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package java.lang;

import java.util.NoSuchElementException;
import java.util.PrimitiveIterator;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.IntConsumer;
import java.util.stream.IntStream;
import java.util.stream.StreamSupport;

/**
 * A <tt>CharSequence</tt> is a readable sequence of <code>char</code> values. This
 * interface provides uniform, read-only access to many different kinds of
 * <code>char</code> sequences.
 * A <code>char</code> value represents a character in the <i>Basic
 * Multilingual Plane (BMP)</i> or a surrogate. Refer to <a
 * href="Character.html#unicode">Unicode Character Representation</a> for details.
 *
 * <p> This interface does not refine the general contracts of the {@link
 * java.lang.Object#equals(java.lang.Object) equals} and {@link
 * java.lang.Object#hashCode() hashCode} methods.  The result of comparing two
 * objects that implement <tt>CharSequence</tt> is therefore, in general,
 * undefined.  Each object may be implemented by a different class, and there
 * is no guarantee that each class will be capable of testing its instances
 * for equality with those of the other.  It is therefore inappropriate to use
 * arbitrary <tt>CharSequence</tt> instances as elements in a set or as keys in
 * a map. </p>
 *
 * @author Mike McCloskey
 * @spec JSR-51
 * @since 1.4
 */

public interface CharSequence {

  /**
   * Returns the length of this character sequence.  The length is the number
   * of 16-bit <code>char</code>s in the sequence.
   *
   * @return the number of <code>char</code>s in this sequence
   */
  int length();

  /**
   * Returns the <code>char</code> value at the specified index.  An index ranges from zero
   * to <tt>length() - 1</tt>.  The first <code>char</code> value of the sequence is at
   * index zero, the next at index one, and so on, as for array
   * indexing.
   *
   * <p>If the <code>char</code> value specified by the index is a
   * <a href="{@docRoot}/java/lang/Character.html#unicode">surrogate</a>, the surrogate
   * value is returned.
   *
   * @param index the index of the <code>char</code> value to be returned
   * @return the specified <code>char</code> value
   * @throws IndexOutOfBoundsException if the <tt>index</tt> argument is negative or not less than
   * <tt>length()</tt>
   */
  char charAt(int index);

  /**
   * Returns a <code>CharSequence</code> that is a subsequence of this sequence.
   * The subsequence starts with the <code>char</code> value at the specified index and
   * ends with the <code>char</code> value at index <tt>end - 1</tt>.  The length
   * (in <code>char</code>s) of the
   * returned sequence is <tt>end - start</tt>, so if <tt>start == end</tt>
   * then an empty sequence is returned.
   *
   * @param start the start index, inclusive
   * @param end the end index, exclusive
   * @return the specified subsequence
   * @throws IndexOutOfBoundsException if <tt>start</tt> or <tt>end</tt> are negative, if
   * <tt>end</tt> is greater than <tt>length()</tt>, or if <tt>start</tt> is greater than
   * <tt>end</tt>
   */
  CharSequence subSequence(int start, int end);

  /**
   * Returns a string containing the characters in this sequence in the same
   * order as this sequence.  The length of the string will be the length of
   * this sequence.
   *
   * @return a string consisting of exactly this sequence of characters
   */
  public String toString();

  /**
   * Returns a stream of {@code int} zero-extending the {@code char} values
   * from this sequence.  Any char which maps to a <a
   * href="{@docRoot}/java/lang/Character.html#unicode">surrogate code
   * point</a> is passed through uninterpreted.
   *
   * <p>If the sequence is mutated while the stream is being read, the
   * result is undefined.
   *
   * @return an IntStream of char values from this sequence
   * @since 1.8
   */
  public default IntStream chars() {
    class CharIterator implements PrimitiveIterator.OfInt {

      int cur = 0;

      public boolean hasNext() {
        return cur < length();
      }

      public int nextInt() {
        if (hasNext()) {
          return charAt(cur++);
        } else {
          throw new NoSuchElementException();
        }
      }

      @Override
      public void forEachRemaining(IntConsumer block) {
        for (; cur < length(); cur++) {
          block.accept(charAt(cur));
        }
      }
    }

    return StreamSupport.intStream(() ->
            Spliterators.spliterator(
                new CharIterator(),
                length(),
                Spliterator.ORDERED),
        Spliterator.SUBSIZED | Spliterator.SIZED | Spliterator.ORDERED,
        false);
  }

  /**
   * Returns a stream of code point values from this sequence.  Any surrogate
   * pairs encountered in the sequence are combined as if by {@linkplain
   * Character#toCodePoint Character.toCodePoint} and the result is passed
   * to the stream. Any other code units, including ordinary BMP characters,
   * unpaired surrogates, and undefined code units, are zero-extended to
   * {@code int} values which are then passed to the stream.
   *
   * <p>If the sequence is mutated while the stream is being read, the result
   * is undefined.
   *
   * @return an IntStream of Unicode code points from this sequence
   * @since 1.8
   */
  public default IntStream codePoints() {
    class CodePointIterator implements PrimitiveIterator.OfInt {

      int cur = 0;

      @Override
      public void forEachRemaining(IntConsumer block) {
        final int length = length();
        int i = cur;
        try {
          while (i < length) {
            char c1 = charAt(i++);
            if (!Character.isHighSurrogate(c1) || i >= length) {
              block.accept(c1);
            } else {
              char c2 = charAt(i);
              if (Character.isLowSurrogate(c2)) {
                i++;
                block.accept(Character.toCodePoint(c1, c2));
              } else {
                block.accept(c1);
              }
            }
          }
        } finally {
          cur = i;
        }
      }

      public boolean hasNext() {
        return cur < length();
      }

      public int nextInt() {
        final int length = length();

        if (cur >= length) {
          throw new NoSuchElementException();
        }
        char c1 = charAt(cur++);
        if (Character.isHighSurrogate(c1) && cur < length) {
          char c2 = charAt(cur);
          if (Character.isLowSurrogate(c2)) {
            cur++;
            return Character.toCodePoint(c1, c2);
          }
        }
        return c1;
      }
    }

    return StreamSupport.intStream(() ->
            Spliterators.spliteratorUnknownSize(
                new CodePointIterator(),
                Spliterator.ORDERED),
        Spliterator.ORDERED,
        false);
  }
}
